This invention relates to micro-electromechanical devices and, in particular, to devices, which are fabricated from several substrates and subjected to anodic bonding during their manufacture.
A number of micro-electromechanical devices, such as accelerometers, pressure sensors, angular rate sensors, inclineometers, etc, are known.
It is well known with such micro-electromechanical devices to provide a glass substrate cover over a semiconductor substrate. Such a cover provides mechanical protection to the device, and can provide a vacuum cavity. The glass can also provide a mount for some of the required electrical components. Placing electrical components on the glass substrate rather than the semiconductor substrate reduces the stray capacitance in the micro-electromechanical device. The glass is bonded to the semiconductor substrate by a process known as anodic bonding and this procedure is known to be harsh and potentially damaging to the components involved.
For example, due to the high temperatures involved, some of the materials may deform so causing defects, which adversely affect the performance of the micro-electromechanical device. Further, stress related effects may disrupt proper functioning of the device after the bonding process.
According to a first aspect of the present invention, there is provided a device comprising electrical and mechanical components comprising multiple layers in which:
a first layer or first set of layers is arranged to function as one or more electrodes or conductors; and
a second layer is arranged to function as one or more press contacts or wire bond pads, wherein the second layer has different physical properties than the first layer or first set of layers, wherein the first layer or set of layers is relatively hard or tough and the second layer is relatively soft or malleable.
Preferably, the first layer or first set of layers is formed from a titanium or, more preferably, from titanium and titanium nitride.
Preferably, the second layer is formed from one of aluminium or gold. The first and second layers may be formed on alkali containing borosilicate glass.
Preferably, the first layer is approximately 7000 xc3x85 thick and the second layer is approximately 5000 xc3x85 thick. The formation, and use of the press contacts as conductors, requires that the thickness of the layers is accurate and reproducible.
The present invention provides electrical and mechanical components which meets the diverse requirements of several functions within the micro-electromechanical device and which can withstand the harsh and potentially damaging environment during anodic bonding. In an embodiment of the invention the components mounted on the glass substrate include capacitor electrodes, conductors, press contacts, and wire bond pads. The conductors are both for electrical connection of the capacitor electrodes and the electric shields. The press contacts form electrical connection to conductors on the mating silicon substrate when the glass and silicon substrates are bonded together by the anodic bonding process.
According to a second aspect of the present invention, there is provided a method of forming electrical and mechanical components in a micro-electromechanical device, the method comprising the steps of:
forming a first layer or set of layers which functions as one or more electrodes or conductors; and
forming a second layer which functions as one or more press contacts or wire bond pads, wherein
the second layer has different physical properties than the first layer or first set of layers, wherein the first layer or set of layers is relatively hard or tough and the second layer is relatively soft or malleable.
Preferably the first and second layers are etched to form the electrodes/conductors or bond pads/press contacts respectively. This process may include photolithography.
Preferably, the hard layer or set of layers is the first layer or set of layers to be deposited on the glass substrate and, subsequently, the second layer is deposited on top of the first layer. The layers may then be subjected to two or more photolithography steps which firstly pattern the wire bond pads and press contacts in to the second layer and, secondly, pattern the conductors and electrodes, as well as, the wire bond pads and press contacts, in to the first layer or set of layers. The pattern for the first layer must include all the features of the second layer because the first layer is under the second layer.
The first layer may be titanium.
The titanium may act as a getter to reduce the gas pressure in a sealed cavity after anodic bonding,and this preferably occurs at approximately room temperature.
Preferably, the titanium layer forms electrodes which remain flat during anodic bonding and which do not form permanent bonds to a flexible silicon structure during anodic bonding. This also applies to the titanium when used as one or more electrical conductors.